Field of the Invention
The present invention relates to an optical head for an optical information recording and reproducing apparatus, such as an opto-magnetic disk apparatus and, more particularly, to an optical head for forming a plurality of light spots on a recording medium. The optical head of the present invention can be applied to an optical information recording/reproducing/erasing apparatus which performs recording and reproduction simultaneously or performs erasure, recording and reproduction simultaneously.
Hitherto, an optical head has been proposed which is capable of simultaneously performing erasure, recording and reproduction, or recording and reproduction, by using a plurality of light spots. One of such proposals is disclosed in, for example, U.S. Pat. No. 4,517,667.
FIG. 1 is a schematic view illustrating such a conventional optical head for generating a plurality of light beams. In FIG. 1, a monolithic semiconductor laser array 1 has a first light emitting point 2a and a second light emitting point 2b. The laser beam emitted from each of the light emitting points 2a and 2b passes through a collimator lens 3, a beam splitter 4, a quarter wavelength plate 5, and an objective lens 6, and is formed into an image on an optical disk 7 as a recording light spot 9a and a reproducing light spot 9b, respectively. The two light spots 9a and 9b are disposed on the same information track 8 on the optical 7 in such a way that the recording light spot 9a is formed before the reproducing light spot 9b. The light beams from the recording light spot 9a and the reproducing light spot 9b pass through the objective lens 6 and the quarter wavelength plate 5, reach the beam splitter 4 and are reflected by this splitter, are condensed by a condenser lens 10, and reach a sensor 11.
An example of an optical system based on what is commonly called a beam size method for detecting a defocused state on the basis of the size of a light spot formed by causing the light spots 9a and 9b to be projected on the sensor 11 is, in principle, shown in FIGS. 2(a) through 2(c). In FIGS. 2(a) through 2(c), the light beam reflected by the optical disk 7 passes through the objective lens 6 and is condensed by the condenser lens 10 on the sensor 11 positioned in front of a focal point 14 of the condenser lens 10. The sensor 11 outputs a signal indicating a defocused state. FIG. 2(b) shows a case in which the distance between the objective lens 6 and the optical disk 7 is equal to the focal length of the objective lens 6, and no defocused state occurs. The light beam condensed by the condenser lens 10 is condensed toward the focal point 14 of the condenser lens 10, and a light spot of a predetermined size is formed on the sensor 11 positioned in front of the focal point 14. FIG. 2(a) shows a case in which the distance between the objective lens 6 and the optical disk 7 is less than the focal length of the objective lens 6, and a defocused state occurs. The light beam condensed by the condenser lens 10 is condensed behind the focal point 14 of the condenser lens 10, and a light spot larger than that of FIG. 2(b) is formed on the sensor 11. FIG. 2(c) shows a case in which the distance between the objective lens 6 and the optical disk 7 is greater than the focal length of the objective lens 6, and a defocused state occurs. The light beam condensed by the condenser lens 10 is condensed in front of the focal point 14 of the condenser lens 10, and a light spot smaller than that of FIG. 2(b) is formed on the sensor 11.
Accordingly, by appropriately positioning the sensor, it is possible to simultaneously record and reproduce information while the light from the two light spots 9a and 9b is detected by the sensor 11 and the defocused state also can be corrected. (Further, any tracking deviation also can be corrected).
However, when the beam size method shown in FIGS. 2(a) through 2(c) is applied to the optical head shown in FIG. 1, which uses a plurality of light spots, the problem discussed below arises.
It is preferable that the distance between the two light spots be as small as possible because (1) the relative angle between two light beams which enter the objective lens desirably should be small in order to optimize the image forming performance of the objective lens 6, and (2) any tracking deviation of the light spot not corrected by a servo, which deviation occurs due to eccentricity of the optical disk or the like, desirably should be small when a defocus or tracking deviation is obtained from either of the two light beams and the servo is performed. On the other hand, generally speaking, since the two light spots on the sensor, as shown in FIGS. 2(a) and 2(c), are not in-focus spots, if the distance between the light spots is decreased, the two light beams on the sensor overlap each other in a portion thereof and interference occurs. When, in particular, a deviation occurs to a greater extent toward the left in FIG. 2(a) due to an adjustment error or the like, the degree of overlapping increases. On the other hand, if the distance between the focus position of the condenser lens and the sensor is decreased to decrease the spot diameter, overlapping of the light beams decreases, but the difference in the distance between the position shown in FIG. 2(b) and the position shown in FIG. 2(c) also decreases, and it becomes impossible to provide the desired defocus detection range.